whats the co level like first,,,,, any sign of a missfire or oil vapour in the burn can cause the mixture to go off,,,,, also if the cat was changed last year without sorting the problem ( the min for a mot is 200ppm but anything over 70ppm means theres a issue) even though the car would have scrapped past the MOT,,,, the following year the cats monolith will be fucked/melted ect,,,,, same counts if you used exhaust past when fitting a cat,,,,, that goes hard and spits pellets of it at the monolith and also kills the cat

get the emissions report and give me a bell in the morning as i used to do hundreds of warrantys on cars that had a cat replaced but the problem never fixed and i would recive the cats back chopped in 2 with the monoliths fucked due to fueling contamination,,,,, they get the emissions report after the new cat is fitted to find the ppm of 140 or sometimes higher...... the car still has a issue just that the government levels are VERY lenient !!